A drill string is a drill pipe that transmits drilling fluid and torque to a drill bit. It can also be associated to an assembly of components such as drill pipe, drill collars, tools, drill bits and the like. Drill string “subs” can refer to the individual tools that perform specific functions when incorporated into a drill string. There are many types of subs, including crossover subs used to change thread types and male/female connection positioning; stabilizer subs used to centralize the drill string during drilling; vibrating subs to reduce the incidence of sticking by maintaining motion in the drill string, and the like.
Typically stabilizers, conditioning subs, vibrating subs and reamers of many types are constructed with one or more ribs, blades, ridges or other features, which protrude from the main body of the sub. These ridges, blades or other protuberances may or may not contain cutters to remove ridges or other irregularities in the borehole. They may also contain additional fluid passages and wear elements to maintain the predetermined diameter as manufactured when the sub is rotating or being pulled out of the borehole or run into the borehole.
Specifically, stabilizers known in the art usually have a plurality of blades which run axially and engage the borehole wall at gage (e.g., at the true diameter of the hole) or near gage. (e.g., close to but not quite as large as the diameter of the hole). Current stabilizers can be straight bladed, can have helix (meaning the blades are spiraled) or can be composed of a plurality of bumps or small surfaces that are configured to contact the borehole wall. FIG. 14 shows a traditional stabilizer, or if the blades had cutting or abrasion elements a traditional reamer as it would look in the hole. Specifically, FIGS. 14 and 16 illustrate a known stabilizer tool 110 having helix blades 102 within hole 114. Stabilizer tool 110 has a threaded pipe connection 116 having external threads 118 at the lead end and internal threads at the trailing end to match external threads 118 of other drill string components.
Known reamers can be fixed (e.g., they are preset at a given diameter) or they can expand to a predetermined diameter. Typical cutting structures for reamers gradually engage the borehole through a gradual expansion of the outside diameter (OD) of the cutters in an arc on the profile of the blade or engagement surface. Reamer apparatuses are usually used for enlarging the borehole or for smoothing out the borehole while drilling oil, gas, water, or geothermal wells. FIG. 17 illustrates a known reamer tool 120 having cutting structures 122.
Known cuttings removal tools, such as shown at 100 in FIGS. 15 and 18, usually have “blades” or “scoops” 102 that pick up cuttings (i.e., the chips of removed rock and the like) which have settled out on the bottom of the long lateral or horizontal sections of a hole 104. A fluid and materials flow path 106 assists in materials removal. The blades work through the rotation of the drill string. Cuttings removal tools are usually used to assist in the drilling of oil, gas, water, or geothermal wells, especially in the long lateral or horizontal wells
Each known types of subs, when employed in Extended Reach Drilling (ERD) applications, produces a restriction to drilled cuttings carried in the circulating fluid by virtue of their protuberances, being oversize relative to the nominal drill string diameter. In many ERD wells, the limits of the drill string, rig pumps, fluid carrying capacity and additional technical limitations are strained. Equipment and physical design limitations are approached on a regular basis.
As ERD wells are, by their nature, non-vertical, there are ledges and ridges created in the drilled surfaces as the borehole is steered in one direction or another. Each time the drilling system is switched from sliding directionally using only mud motor RPM to drilling ahead and rotating the entire string, there is a change in borehole drilled diameter. The concatenation (i.e., a series of interconnected or events) of these changes increases borehole rugosity and overall friction that reduces efficiencies of the drill bit, the motor torque and both the axial and torsional movement of the drill string during operations.
Additionally, as wellbores lengthen, the ability of the fluid to move cuttings from the wellbore to the surface declines, this movement being a function of the pump output, mud properties, temperature, borehole rugosity and the like. Typically, gravity pulls the drill string to the lower boundary of the wellbore, which forces a larger volume of the fluid into the larger annular area created with the drill string on the bottom of the wellbore. Cuttings thus tend to build up along the bottom of non-vertical wellbores, decreasing drilling efficiency by increasing torque values (both axial and torsional). This increase in torque increases the required Mechanical Specific Energy (MSE), which is defined as the amount of energy required to fail (e.g., cut through) a unit volume of rock.
Irregular borehole surfaces (rugosity) in the curve section of a wellbore (portion of the wellbore where the departure from vertical is initiated) and accumulation of cuttings in the lateral (the portion of the well where the angle approaches horizontal or more) have a well defined, detrimental effect on drilling efficiency, requiring an increase in MSE to successfully drill to the desired borehole length.
Thus the nature of ERD wells is that they typically reach or approach the limits of mechanical rig operational capacities, mud properties and downhole tool abilities. Any tool or item that serves to increase efficiencies downhole will reduce the MSE required and improve the outcome of the well being drilled as planned. Accumulation of drilled cuttings on the lower circumference of the borehole reduces efficiencies in many ways, and raises required MSE needed to drill ahead.
In ERD wells, adding additional fluid openings reduces volume at the bottom (bit exit) of the drill string unless the pump output is increased to compensate; it is a closed system with finite input. When the input limits are approached or reached in typical ERD wells, adding any hydraulic opening away from the drill bit reduces the fluid velocity, and thus reduces cleaning and carrying capacity of the drilling mud throughout the wellbore. It is thus counterproductive to add additional fluidic enhancement elements in most ERD wells without concomitantly increasing fluid volume. This precludes pulsing or hydraulically driven vibrating subs, which become more problematic in operation as the wellbore lengthens and the available hydraulic energy declines.